Since cecropins, which are novel antimicrobial peptides from silkworm larva, were found in the results of research on defense mechanisms of insects against the microbial invasion, the importance of novel peptides as bioactive substances has been greatly recognized. The recent results of several years of research show that almost all higher organisms accumulate or secrete antimicrobial peptides in their bodies as defense measures against pathogenic microbes, separate from immune systems. Approximately 2,000 antimicrobial peptides have been found so far, and these peptides have been known to have different amino acid compositions in every discovered species but have similar mechanisms of action. Representative antimicrobial peptides known in the art include cecropins, magainins, bombinin, defensins, tachyplesin, and buforin. It was known that these antimicrobial peptides commonly consist of 17 to 24 amino acids, and have antimicrobial activity against prokaryotes or fungi as well as gram-negative and gram-positive bacteria, and are effective against cancer cells and viruses as well.
In particular, among the antimicrobial peptides, it was reported that a magainin is a peptide that has a composition of 23 amino acids and is separated from the skin of an amphibian (Zasloff, M., Proc. Natl. Acad. Sci. USA, 84, pp 5449-5453, 1987), and acts on human lung cancer cells as well as pathogens. Also, most of the antimicrobial peptides specifically act on cells to rapidly kill target cells, and largely exhibit activity spectra in a wide range (Park, C. B. et al., Biochem. Biophys. Res. Comm., 218, pp 408-413, 1996).
The antimicrobial peptides have advantages in that they 1) have potent antimicrobial activity against a wide range of microbes, 2) act on only pathogens invading from the outside without destroying host cells, and thus act as an antimicrobial substance that is not harmful to human bodies, 3) have a low probability to induce resistance because they have a mechanism of action completely different from those of the conventional antimicrobials which induce resistance in microbes, which was problematic, 4) may be mass-produced through gene manipulation because they have no secondary modification such as glycosylation, and 5) are highly industrially usable in the field of pharmaceuticals and food because they show high physicochemical stability with respect to heat, acids or alkalis.
The currently reported mechanisms of action of the antimicrobial peptides are mainly divided into two categories. First, a majority of the antimicrobial peptides have a mechanism of action in which the antimicrobial peptides increase the permeability of bacterial cell membranes to destroy a membrane potential and terminate cell metabolism. Second, a minority of the antimicrobial peptides have a potent mechanism of action in which the antimicrobial peptides invade bacterial cells and bind to DNA or RNA to inhibit transcription or translation.
Structural elements which are known to be important for these activities of the antimicrobial peptides may include the following: 1) an amphipathic helix, 2) a distribution of residues to stabilize the helix, 3) a distribution of basic residues, 5) a distribution of hydrophobic residues, 5) an interaction between charged residues and a dipole of the helix, and 6) a salt bridge between oppositely-charged residues.
Meanwhile, lactophoricin (LPcin-I) present in cow milk is an cationic and amphipathic peptide that consists of 23 amino acid residues, and corresponds to a carboxyl terminal 113 to 135 region of PP3. LPcin-I inhibits the growth of both gram-positive and gram-negative bacteria, but does not have a hemolytic action at a concentration of 200 μM or less. Unlike LPcin-I, LPcin-II corresponding to a 119th to 135th amino acid region of PP3 is known to have no antimicrobial functionality.
However, to commercialize the LPcin-I known to have an antimicrobial ability, there is an urgent demand to develop technology in which antimicrobial peptides consisting of shorter amino acid sequences are prepared so as to exhibit a higher antimicrobial ability than the wild-type LPcin-I and reduce production costs.